IPT uses a varying magnetic field to couple power across an air gap, to a load, without physical contact. The air gap is present between a primary conductor such as an elongate loop of conductive material (generally referred to in this document as a track), and one or more pick-up devices that have a secondary coil which receive power from the magnetic field associated with the track. System performance is not affected by wet or dirty environments and there are no safety risks under such conditions since the components are completely isolated. IPT is reliable and maintenance free unlike conventional plug or brush and bar contact based methods such as those used on trams and electric buses. IPT is presently used in numerous industrial applications such as materials handling and integrated circuit fabrication. The systems vary in capacity from 1 W-200 kW and can be used to both power and recharge robots, Automatic Guided Vehicles (AGV), electronic devices, recreational people movers, buses and Electric Vehicles (EVs). IPT systems may be divided into two distinct types: distributed systems that consist of one or more movable loads that may be placed anywhere on a track, and lumped systems that only allow power transfer at a defined location.
Distributed systems are particularly suited to Roadway Powered EV (RPEV) applications, however practical large scale RPEV systems have so far been infeasible. This is due to the large horizontal (˜700 mm) tolerance and ground clearance (150-200 mm) required by unguided EVs. The track topology presented in this document offers a significant improvement over previous designs by allowing increased horizontal tolerance with minimal increase to the system cost. Those skilled in the art will appreciate that this document refers to application of the invention in the context of EVs, but the invention is applicable to many other IPT system applications.
EVs help reduce dependence on fossil fuels, emission of greenhouse gasses and emission of pollutants. Consequently, uptake of EVs has been increasing since the 1990's however market penetration has been low because EVs are not as cost effective as conventional vehicles. The present EV market is dominated by hybrid vehicles that derive their energy from a combustion engine, however, plug-in EVs (PHEV) have recently been introduced enabling energy from the grid to mitigate gasoline consumption. In order for EVs to gain widespread adoption, major improvements are required in battery life and cost, and grid connection. The latter allows opportunistic charging after each trip rather than a long charge at the end of the day. As a result battery wear is significantly reduced by minimising the depth of discharge and the EV has a lower cost since a smaller battery is required. The preferred solution, that makes EVs more cost effective than gasoline vehicles is to power and recharge the EV via the road. It should be noted that the infrastructure for such a dynamic charging system could be relatively small because travel on interstate highways makes up 1% of roadway miles but carries 22% of all vehicle miles travelled. An EV that has 50% of its driven miles connected to a dynamic charging system would be as cost effective as a conventional vehicle and does not incur additional gasoline costs.
An IPT system comprises three main components that are shown for a single phase system in FIG. 1. The power supply produces a sinusoidal current (typically in the 10-100 kHz frequency range for medium to high power systems) that drives a current (I1) in an inductive primary conductive path, or track. The parallel compensation capacitor C1 allows the track current, I1, to resonate, increasing the magnetic field strength in the vicinity of the track. This minimises the VA rating of the power supply for a given load. The track and secondary receiver device to which power from the track is transferred inductively act as a loosely coupled transformer enabling power transfer over relatively large air gaps. The receiver is commonly referred to as an IPT Pick-up (PU) and has an inductance, L2, which is tuned for resonance at the frequency of the current in the track using C2. This compensates for the relatively large PU leakage inductance. The voltage across C2 is rectified and a switched mode controller enables the resonant tank to operate at a defined quality factor, Q, to boost power transfer and provide a usable DC output. The power output of an IPT system (Pout) is quantified by the open circuit voltage (Voc) and short circuit current (Isc) of the PU as well as the PU circuit quality factor, as shown in (1).
                              P          out                =                                            P              su                        *            Q                    =                                                    V                oc                            *                              I                sc                            *              Q                        =                                          ω                ⁢                                                                  ⁢                                  MI                  1                                *                                                      MI                    1                                                        L                    2                                                  *                Q                            =                              ω                ⁢                                                                  ⁢                                  I                  1                  2                                ⁢                                                      M                    2                                                        L                    2                                                  ⁢                Q                                                                        (        1        )            
Psu is the uncompensated power, ω is the angular frequency of the track current I1, M is the mutual inductance between the track and PU. As shown in (1), the output power is dependent on the power supply (ωl12), magnetic coupling (M2/L2) and PU controller (Q). Increasing the power output and separation between the track and PU is highly desirable but efficiency is limited by the operational frequency (switching loss) and current rating (copper loss) of the system. Allowing a system to operate at a high Q boosts power transfer but in practical applications it is normally designed to operate between 4 and 6 due to component VA ratings and tolerances. Due to these limits, the greatest increase in system performance can be achieved by good magnetic design.
A laboratory 16 kW prototype single phase RPEV system has been built in the past (see G. A. Covic, J. T. Boys, M. L. G. Kissin and H. G. Lu, “A Three-Phase Inductive Power Transfer System for Roadway-Powered Vehicles”, Industrial Electronics, IEEE Transactions on, vol. 54, no, 6, pp. 3370-3378, 2007). The track in that system is essentially an elongated spiral winding with the longer sides placed on adjacent lanes. Consequently the PU is only exposed to flux generated by current flowing in one direction and there are no nulls in the power profile across the track as would occur with a simple PU on a conventional single phase track, where the conductors are placed side by side. The system was built before modern ferrites and power electronic components were developed, and this is reflected in its performance. The air gap between the track and PU was controlled by an electronic actuator to be 30 mm and full power could be supplied up to an offset of 120 nm from the track centre. The relatively low horizontal tolerance necessitated an automatic guidance system.
A 5 kW single phase system that operates with a 200 mm air gap has been built and tested and is disclosed in G. A. Elliott, J. T. Boys and A. W. Green, “Magnetically coupled systems for power transfer to electric vehicles,” in Proceedings International Conference on Power Electronics and Drive Systems, Singapore, 1995, pp. 797-801. However the horizontal tolerance is 60 mm. Notably, the system does not use ferrite in the PU and this causes significant problems when installed in an EV. Ferrite ensures the flux remains within the PU and allows aluminium shielding to be used, this is necessary to limit losses in the steel chassis and to meet magnetic field exposure guidelines.
One approach discussed but not employed by G. Elliott, S. Raabe, G. Covic and J. Boys, in “Multi-Phase Pick-Ups for Large Lateral Tolerance Contactless Power Transfer Systems,” Industrial Electronics, IEEE Transactions on, vol. PP, no. 99, pp. 1-1, 2009 to improve the horizontal tolerance on a single phase track was to use a complicated PU that contained six offset coils. As the PU is moved horizontally across the track different sets of coils are energised thus increasing tolerance. However this approach is not suitable for high power systems due to mutual coupling between the coils that makes tuning the active coil problematic. The unused parallel tuned coils need to be shorted and doing so affects the flux path of the active coil resulting in losses.
In order to improve horizontal tolerance, a three phase track topology as shown in FIG. 2(a) was proposed by G. A. Covic, J. T. Boys, M. L. G. Kissin and H. G. Lu, in “A Three-Phase Inductive Power Transfer System for Roadway-Powered Vehicles,” Industrial Electronics, IEEE Transactions on, vol. 54, no. 6, pp. 3370-3378, 2007. The vehicle drives along the length of the track, Tx, which is referred to as the x-axis. The system uses an inductor-capacitor-inductor (LCL) impedance converting network that converts the voltage sourced inverter into a current source suitable for driving the inductive track. The leakage inductance of the isolating transformer is used as the first inductor and the track forms the last inductor, so that only real power passes through the transformer. Large reactive currents (I1 in FIG. 1) circulate in the track and capacitor only. Three individual isolating transformers connected in a delta-delta configuration were used for each phase, however the output terminals of the transformers were connected directly to the start and return of each track loop resulting in a six wire track. This track topology is termed “bipolar” in this document because the PU is exposed to both forward and returning currents to the supply. The overlapping nature of the track phases results in currents that differ by 60° in each adjacent wire and in a similar manner to windings in a cage induction motor, this creates a travelling field across the width (Ty) of the track. This moving field results in a wide and even power profile with a simple single coil PU. This power profile can be further improved as discussed in “Multi-Phase Pick-Ups for Large Lateral Tolerance Contactless Power Transfer Systems,” Industrial Electronics, IEEE Transactions on, vol. PP, no. 99, pp. 1-1, 2009 using a PU with quadrature coils.
However, a consequence of having overlapping tracks is the presence of mutual inductance between phases, so that energy from one track phase couples into adjacent phases, similar to the power coupling between each track conductor and the PU. This cross coupling causes different legs in the inverter to source large currents and the DC bus voltage surges undesirably as energy is fed into the inverter.